The present invention relates to polyester products such as poly(ethylene terephthalate) containers. More particularly, this invention relates to reduction of acetaldehyde content of melt-processed polyesters by incorporation of additives capable of reacting with acetaldehyde.
Polyesters, especially poly(ethylene terephthalate) (PET) are versatile polymers that enjoy wide applicability as fibers, films, and three-dimensional structures. A particularly important application for PET is for containers, especially for food and beverages. This application has seen enormous growth over the last 20 years, and continues to enjoy increasing popularity. Despite this growth, PET has some fundamental limitations that restrict its applicability. One such limitation is its tendency to generate acetaldehyde (AA) when it is melt processed. Because AA is a small molecule, AA generated during melt processing can migrate through the PET. When PET is processed into a container, AA will migrate over time to the interior of the container. Although AA is a naturally occurring flavorant in a number of beverages and food products, for many products, the taste imparted by AA is considered undesirable. For instance, AA will impart a fruity flavor to water, which detracts from the clean taste desired for this product.
PET is traditionally produced by the transesterification or esterification/polymerization of a terephthalate precursor (either dimethyl terephthalate or terephthalic acid) and ethylene glycol. If the end use application for the melt-polymerized PET is for food packaging, the PET is then subjected to a second operation known as solid-state polymerization (SSP), whereby the molecular weight is increased and the AA generated during melt processing is removed. A widely used method to convert the solid-state polymerized PET into containers consists of drying and remelting the PET, injection molding the polymer into a container preform, and subsequently stretch blow-molding the preform into the final container shape. It is during the remelting of the PET to fashion the container preforms that AA is regenerated. Typical preform AA levels for PET processed in this manner using the most modern injection molding equipment is 6-8 xcexcg/g (ppm).
Historically, the impact of AA on product taste has been minimized by careful control of the melt processing conditions used to make containers or preforms, and by use of special processing conditions in polymer preparation. This approach is successful for most packages, where the taste threshold for AA is sufficiently high, or where the useful life of the container is sufficiently short. However, obtaining low preform AA carries with it a significant cost. That cost includes the need to carry out a separate processing step after the melt polymerization of PET (solid-state polymerization), the need for specially designed injection molding equipment, and the need to continually monitor the preform AA content during container production. For other applications, where the desired shelf-life of the container is longer, the product is more sensitive to off-taste from AA, or the prevailing environmental conditions are warmer, it is not possible to keep the beverage AA level below the taste threshold by using these methods. For example, in water the taste threshold is considered to be less than about 40 xcexcg/L (ppb), and often a shelf-life of up to two years is desired. For a PET bottle that contains about 600 ml of beverage, a preform AA content of 8 ppm can result in a beverage AA level greater than 40 ppb in as little as one month.
In addition to careful control of melt-processing conditions for PET, prior art methods include modifications to the PET itself via use of lower intrinsic viscosity (IV) resins and/or the use of lower melting PET resins. However, each of these resin modification approaches has been only partially successful, and they suffer from their own limitations. For example, lower IV resins produce containers that are less resistant to environmental factors such as stress crack failure. Lower melting resins are achieved by increasing the copolymer content of the PET resin. However, increasing the copolymer content of the resin also increases the stretch ratio of the PET, which translates into decreased productivity in injection molding and blow molding.
Another prior art approach has been to incorporate additives into PET that will selectively react with, or scavenge, the AA that is generated. Thus, Igarashi (U.S. Pat. No. 4,837,115) discloses the use of amine-group terminated polyamides and amine-group containing small molecules. Igarashi teaches that the amine groups are effective because they can react with AA to form imines, where the amine nitrogen replaces the oxygen in the AA carbonyl group. Igarashi teaches that essentially any amine is effective. Mills (U.S. Pat. Nos. 5,258,233; 5,650,469; and 5,340,884) and Long (U.S. Pat. No. 5,266,416) claim the use of various polyamides, especially low molecular weight polyamides. Turner and Nicely (WO 97/28218) claim the use of polyesteramides. These polyamides and polyesteramides are believed to react with AA in the same manner as described by Igarashi.
While these AA scavengers are effective at reducing the AA content of melt-processed PET, they also suffer from drawbacks. In particular, relatively high loadings of the polyamides are needed to effect significant AA reductions, and a very significant yellowing of the PET occurs on incorporation of these amine-containing additives. This color formation is believed to be due to the color of the imine group itself, and is thus unavoidable. The yellow color formation inherently limits this approach to articles where the PET can be tinted to mask the color. Unfortunately, most PET articles in use today are clear and uncolored.
Another prior art approach of AA scavenger as disclosed in U.S. Pat. No. 6,274,212 uses a thermally stable organic additive, which upon reaction with AA forms an unbridged 5 or 6 member ring structure and water. As there is no inherent color formation resulting from the unbridged 5 or 6 member ring structure, the AA scavengers of this invention have much better color as compared to the above amide or amine based additives. The organic additives disclosed in the patent possess at least two hydrogen substituted heteroatoms bonded to carbons of the organic additive compound and are reactive with acetaldehyde in the polyester to form water and a resulting organic compound comprising an unbridged 5 or 6 member ring which includes the at least two heteroatoms. These additives are very effective in sequestering AA. However, due to the relatively high vapor pressure of the preferred additives, such as anthranilamide, there is some loss of the scavenger in the injection molding process due to vaporization. This relatively high vapor pressure can also lead to mold deposits in the preform forming process, which results in the need for more frequent mold cleaning and maintenance. For the same reason, the incorporation of the additive into PET masterbatches is relatively ineffective, since the additive tends to be lost from the PET during drying.
Another potential issue with the proffered additives in the patent disclosure is their ability to migrate. The preferred molecules are of relatively low molecular weight, which tend to have higher extraction rates than desired. Therefore, although these AA scavengers have be used successfully in decreasing the preform and beverage AA content on molding solid state polymerized resins, they are less applicable if the starting AA level is much higher and much higher loadings of AA scavengers are needed. Such is the case when a preform is formed from a PET melt directly from melt-phase polymerization, without intermediate pelletization and solid state polymerization.
Therefore, although the small molecule AA scavengers as covered by U.S. Pat. No. 6,274,212 are effective, there is a need for improved AA scavengers.
This invention addresses the above described need by providing a method for decreasing the acetaldehyde content of melt processed polyester comprising combining at least one organic additive compound with polyester. The organic additive compounds of the present invention comprise at least two component molecular fragments, each component molecular fragment comprising at least two hydrogen substituted heteroatoms bonded to carbons of the respective component molecular fragment. The component molecular fragments of the organic additive compound are each reactive with acetaldehyde in the polyester to form water and a resulting organic molecular fragment comprising an unbridged five or six member ring including the at least two heteroatoms. The resulting organic fragments do not impart an off-taste to beverages packaged in containers made with this treated polyester, and do not significantly discolor the polyester. Combining these organic additive compounds with the polyester require no special equipment or processing steps, and the organic additive compounds are substantially thermally stable at the melt processing temperature of the polyester.
The organic additive compounds have at least twice the molecular weight of the component molecular fragments alone and, as a result, are much less extractable from polyester, and can therefore be used at higher loadings. Furthermore, since the organic additive compounds can be used at higher loadings, the additive compounds can be used to decrease the AA content of melt-phase PET resin to acceptable levels, thus enabling the practice of direct performing, without the need for an intermediate solid-state polymerization step.
In addition, the organic additive compounds described above have a markedly lower vapor pressure than the corresponding component molecular fragments, and therefore vaporize much less readily. For this reason, they do not plate out on tooling and molds during the formation of polyester articles. The low vapor pressure and high thermal stability also enable the additive compounds to be incorporated in polyester resin and retain their activity through solid-state polymerization and subsequent drying. Because of the thermal stability and low volatility of the additives of the present invention, the point at which they are added to polyesters are not particularly critical. Thus, to reduce the AA content of preforms made from solid-state polymerized PET, the organic additive compounds can be added at the end of the melt-polymerization step, as a masterbatch to the SSP resin prior to drying, or at the point of remelting the SSP resin immediately prior to injection molding.
The polyester can be combined with the organic additive compound at different stages of polyester processing. In one embodiment, the polyester can be combined with the organic additive compound by preparing a polyester melt and combining the additive compound with the polyester melt. The additive compound can be added to the polyester prior to the melt-processing or after the melt-processing. In one embodiment, the polyester melt is not solidified prior to forming a polyester article.
This invention also encompasses compositions for use in making polyester articles with decreased acetaldehyde content comprising polyester and the above described organic additive compounds, polyester articles made with the organic additive compounds described above, and a corresponding method for making polyester articles.
Furthermore, this invention encompasses containers and container preforms made with the above described composition and packaged beverages comprising a beverage disposed in such a container.